function trickyFunction(a) {
    a = a || 42;
    let result = (function(a) {
        return a * 2;
    })(a);
    return result;
}

console.log(trickyFunction(0));

let x = 1;
function outer() {
  let x = 2;
  function inner() {
    console.log(x);
    let x = 3;
  }
  inner();
}
outer();

const user = {
  name: 'John',
  greet() {
    const getName = () => {
      console.log(this.name);
    };
    getName();
  },
  farewell: function() {
    function getName() {
      console.log(this.name);
    }
    getName();
  }
};

user.greet();
user.farewell();

const arr = [1, 2, 3, 4, 5];
const result = arr
  .map(x => x * 2)
  .filter(x => x > 5)
  .reduce((acc, val) => {
    return acc + (val % 3 === 0 ? val : 0);
  }, 0);

console.log(result);

const items = new WeakSet();
const obj1 = { id: 1 };
const obj2 = { id: 2 };

items.add(obj1);
items.add(obj2);

obj2.value = 'test';
console.log(items.has(obj2));

obj2 = null;
setTimeout(() => {
  console.log(items.has(obj2));
}, 0);

function* range(start, end) {
  let current = start;
  while (current <= end) {
    if (current % 3 === 0) {
      current++;
      continue;
    }
    yield current++;
  }
}

const gen = range(4, 10);
const result = [...gen];
console.log(result);

async function demo() {
  console.log('1');
  
  setTimeout(() => console.log('2'), 0);
  
  Promise.resolve().then(() => {
    console.log('3');
    setTimeout(() => console.log('4'), 0);
  });
  
  await Promise.resolve();
  console.log('5');
  
  queueMicrotask(() => console.log('6'));
}

demo();
console.log('7');